diff --git a/pkgs/contract-profile/main.rkt b/pkgs/contract-profile/main.rkt
index bdd28ca657..6204c09d08 100644
--- a/pkgs/contract-profile/main.rkt
+++ b/pkgs/contract-profile/main.rkt
@@ -31,6 +31,12 @@
                      (blame-swap b) ; swap back
                      b))))
   (define regular-profile (analyze-samples samples*))
+  ;; all blames must be complete, otherwise we get bogus profiles
+  (for ([b (in-list all-blames)])
+    (unless (and (blame-positive b)
+                 (blame-negative b))
+      (error (string-append "contract-profile: incomplete blame:\n"
+                            (format-blame b)))))
   (contract-profile
    total-time live-contract-samples all-blames regular-profile))
 
diff --git a/pkgs/contract-profile/utils.rkt b/pkgs/contract-profile/utils.rkt
index 0e9463f665..014a8e644c 100644
--- a/pkgs/contract-profile/utils.rkt
+++ b/pkgs/contract-profile/utils.rkt
@@ -35,8 +35,8 @@
         (lambda () body ...))))
 
 ;; for debugging
-(define (print-blame b)
-  (printf (string-append "#<blame positive=~a\n"
+(define (format-blame b)
+  (format (string-append "#<blame positive=~a\n"
                          "        negative=~a\n"
                          "        contract=~a\n"
                          "        value=~a\n"